1. Field of the Invention
The present invention relates to switching power conversion circuits, including but not limited to, bi-directional power conversion circuits.
2. Description of Related Art
Bi-directional, isolated DC-to-DC inverters are useful in a wide range of applications. Examples of such applications include, but are not limited to, uninterruptible power supplies, battery charging systems, auxiliary power supplies for hybrid electrical vehicles and pure electric vehicles.
FIG. 1 depicts a basic two inductor bi-directional DC-to-DC converter. In a boost mode, switches Q1 and Q2 operate as the main switching device to transfer power from Vlo to Vhi. In a buck mode, switches S1-S4 operate as the main switching device to transfer power from Vhi to Vlo.
It has been recognized that two-inductor boost converters suffer from limited output voltage regulation range when operated at low duty cycles. More particularly, when a load is below the minimum input power of a converter circuit of this type, further decreases in the load result in abnormal increases in output voltage due to excess energy storage in the inductors. Yan et al. have proposed a solution to avoid these increases in output voltage. (“Isolated Two-inductor Boost Converter with One Magnetic Core,” Eighteenth Annual Applied Power Electronics Conference and Exposition, Feb. 9-13, 2003, Miami Beach, Fla., pp. 879-885.) An auxiliary transformer is utilized in series with two inductors to magnetically couple the two input current paths, ensuring that the current in the two inductors is the same. Thus, inductor current is eliminated when the load draws no current. A magnetic component disclosed by Yan et al. provides an isolated two-inductor boost converter with one transformer. This component has two inductor windings intrinsically coupled and is implemented with one gap in a three-leg magnetic core. However, circuits utilizing an auxiliary transformer and those utilizing the magnetic component of Yan et al. may require that these windings or components be capable of carrying high currents.
Another example of a DC-to-DC converter is disclosed by Li et al. (“A Natural ZVS High-power Bi-direction dc-to-dc Converter with Minimum Number of Devices,” presented at IEEE Industry Applications Society Annual Meeting, Sep. 30-Oct. 4, 2001, Chicago, Ill., pp. 1874-1881.) This converter is operated with dual half-bridges placed on each side of an isolation transformer. When power flows from the low-voltage side to the high-voltage side, the circuit operates in boost mode. Conversely, when power flows in the opposite direction, the circuit works in buck mode to recharge a battery used to provide power to the low-voltage section. To support bidirectionality, a complex thirteen-step commutation procedure is used that depends upon the magnitudes of various currents at specified times.
FIG. 2 depicts the converter described in U.S. Ser. No. 10/881,213, assigned to the same assignee as the present application titled “DC Converter Circuit with Overshoot Protection.” The additional capacitors C1 and C2 provide soft switching for S2 and S4 when S1-S4 are working as main switching device for the buck mode, and to provide overshoot protection for Q1 and Q2 when Q1 and Q2 are working as main switching device for the boost mode.
FIG. 3 shows a gate drive control waveform described in U.S. Patent Publication No. 2005/0024904A1 of Kajouke et al., U.S. Ser. No. 10/630,496, assigned to the same assignee as the present application. This publication describes a control method that in general can apply to any bi-directional converter including the converters shown in FIG. 1 and FIG. 2 to achieve no-load operation in either direction. When this method is applied to the circuit shown in FIG. 2, it provides the no-load operation in the buck mode (power flowing Vhi to Vlo). However when this method is applied to the boost mode (power flowing from Vlo to Vhi) the overshoot protection provided by C1 and C2 for Q1 and Q2 is destroyed by the switching action of S2 and S4 which is needed only for the no-load operation.
In the normal operation of the circuit shown in FIG. 2 operating in boost mode, switches Q1 and Q2 work as the main switching device, and switches S1-S4 work as rectifiers with the internal anti-parallel diodes. However, when operating in a no load condition, a minimum amount energy will have to flow from power source Vlo to Vhi because both Q1 and Q2 can not be turned off at the same time. This condition is needed to avoid abrupt changes in current flowing through inductors that can lead to destructive voltage spikes across the switches. If even a minimal amount of energy were to flow from power source Vlo to Vhi and there was no reverse energy sent back from Vhi to Vlo, the output voltage across capacitor C0 would continue to increase, and thus, could not be regulated. The method proposed by Kajouke et al., U.S. Ser. No. 10/630,496, uses switches S1-S4 to provide reverse energy flow to balance out minimal amount of energy flowing forward from power source Vlo to Vhi. However, the switching action of switches S2 and S4 will destroy the overshoot protection condition for switches Q1 and Q2.
What is needed is a circuit and method of operating the circuit that will provide voltage regulation at a no-load condition over a wide voltage range and also will provide overshoot protection for Q1 and Q2.